Intracranial atherosclerosis is responsible for 50,000 ischemic strokes each year in the USA. Noninvasive testing such as transcranial Doppler ultrasound (TCD) and magnetic resonance angiography (MRA) to identify intracranial atherosclerosis is in widespread use but has not been rigorously validated against the gold standard, catheter angiography. The recently NIH-funded WASID trial (Warfarin Aspirin Symptomatic Intracranial Disease) will compare warfarin with aspirin for stroke prevention in patients with intracranial atherosclerosis. WASID requires performance of angiography along with TCD and MRA, providing an opportunity to critically evaluate these nonivasive tests. Main Objective. The purpose of SONIA is to validate the noninvasive diagnosis of intracranial atherosclerosis. The primary aim of SONIA is to define velocity values on TCD and anatomic abnormalities on MRA that identify severe (50-99 percent) intracranial stenosis or large, proximal arteries seen on catheter angiography. SONIA will define the criteria, or cutpoints, for an abnormal TCD or MRA and show that they perform with a reliable positive predictive value (PPV). Study Design. SONIA will be conducted in collaboration with WASID. Study-wide cutpoints defining positive TCD and MRA have been developed and reviewed by the site investigators of WASID. Hard copy angiography, TCD and MRA generated in WASID will be centrally read in SONIA. The performance of TCD and MRA will be monitored and analyzed in SONIA to demonstrate cutpoints that achieve a PPV of 80 percent for the identification of severe intracranial stenosis on angiography. Conclusions. Central readings will be used to validate the cutpoints that function with a PPV of 80 percent and to develop measures of negative predictive value (NPV), inter- and intra-observer variability. Sensitivity and specificity will be determined after adjustment for verification bias and employed in receiver-operator characteristic analyses. SONIA will use these techniques to develop TCD or MRA cutpoints that minimize the clinical consequences of test errors that occur in the noninvasive evaluation of patients with suspected intracranial atherosclerosis.